Laser ranging systems that optically determine the distance to an object are well adapted for use as precise metrology equipment. For example, such precise metrology equipment is necessary to control or check industrial processes requiring accurate machining or fabrication of parts to tight tolerances. One example of such a laser ranging system is a variable depth range camera as described in U.S. Pat. Nos. 4,963,017; 5,032,023; 5,061,062; and 5,082,362, each of which is assigned to the assignee of the present invention and which are incorporated herein by reference.
One of the problems faced by developers and users of precise metrology equipment is calibration drift due to thermal expansion effects and equipment wear. These difficulties have necessitated the use of elaborate environmental controls to maintain the precision of the metrology equipment, such as the installation of laser ranging equipment in temperature controlled rooms. The costs to build and operate such environmental equipment increases fabrication costs; further, the limited mobility of such equipment can impede the fabrication process.
Even with stable environmental conditions, precision metrology equipment requires downtime for periodic calibration. Such calibration typically involves the measurement of a known object and comparison of the results of the measurement with previously obtained results; such a procedure is both time consuming and impractical for metrology devices having large working volumes (or ranges of measurement) due to the costs associated with designing, building, and storing large precision surfaces for use in calibration runs. Another type of calibration system employs a target on a rail; the target moves along a path within the depth of field of the laser ranging system. Comparing the laser ranging system's range determination with the simultaneously-determined range values obtained with an interferometer coupled to the rail system provides calibration data. Because this type of calibration system must have one dimension as long as the longest range that is to be used in the calibration process, as a practical matter they are limited to small depths of range and, because of their size and associated difficulty in moving and storage, do not lend themselves to frequent calibration of the metrology equipment. Further, aside from the time and expense of having the equipment out of service during the calibration operation, infrequent calibration runs raises the concern the instrument has become uncalibrated during a period when it is used for production runs.
The development of non-contact laser ranging equipment, such as the variable depth range camera noted above, having a large working volume and that is well adapted for use in a shop-floor environment has led to a need for a calibration system that is fast. The calibration system desirably is both fast and readily attached to the laser ranging system to enable the laser ranging system to be recalibrated at short intervals (e.g., every 5 to 10 minutes, or between each measurement series in the fabrication of a workpiece). The calibration system is also desirably compact and rugged to permit reliable use in a manufacturing environment.